The present invention relates to a method and apparatus for laser processing of a semiconductor substrate, more specifically, to the high quality laser cutting of monocrystalline and polycrystalline silicon resulting in near-zero kerf cuts with low total roughness, through the use of a standard wavelength laser.
Silicon is transparent to light having a wavelength greater than 1.3 μm. Since the most common lasers (such as YAG lasers) operate in the 1064 nm wavelength range, these lasers produce light that is partially absorbed by silicon and thus are unable to cleanly cut silicon by burst ultrafast pulsed laser induced filamentation techniques owing to the significant linear absorption of the laser pulses. However, they can ablatively laser cut silicon at this wavelength. The same laser parameters and apparatus can be used to cut glass or sapphire via filamentation. Using a tightly focused beam inside the substrate causing optical break down to dice Si thin wafers is also known as stealth dicing invented and developed by Hamamatsu. See U.S. Pat. No. 7,396,742.
To cut silicon by filamentation requires building a laser that operates at a wavelength above 1.3 μm. Alternatively, a method of altering or modifying the 1064 nm wavelength laser beam into a beam of the correct wavelength must be used. One method is to use an optical parametric generator (OPG), or optical parametric amplifier (OPA) or white light generation in an optical Kerr medium.
In optical parametric generation, an input light beam of a given frequency is down converted into two light beams at lower frequency. These two lower-frequency beams are called the “signal” and “idler”.
There is a huge demand for laser processing (orifice drilling or cutting) of materials such as silicon. The applications are wide ranging and include such things as semiconductors, microelectronics, filters for air monitoring, particle monitors, cytology, chemotaxis, bioassays, and such, and commonly require very clean, very uniform orifices a few hundred nanometers to tens of micrometers, in diameter. The prior art ablative methods of machining of Silicon with lasers having wavelengths less than 1.3 μm leave very rough cut surfaces, compromise the material with micro cracks, create surface ejecta mounds, create stress zones and are prone to causing large regions of collateral thermal damage. While laser ablation processes can be dramatically improved by selecting lasers with wavelengths that are strongly absorbed by the medium (for example, deep UV excimer lasers or far-infrared CO2 laser), the above disadvantages cannot be eliminated due to the aggressive interactions inherent in this physical ablation process. Further, the prior art laser ablation systems have low throughput times, and cannot attain as close a tolerance machining as can be achieved by the present method.
The present method and apparatus allows for improved processing especially scribing and cutting of silicon, by filamentation by burst ultrafast laser pulses, without the need for a specially built laser that outputs 1.3 μm and greater wavelength. This new invention utilizes and combines known and new technologies in a unique and novel configuration to overcome the aforementioned problems and accomplish this and provides enough confidence and proof to invest in building new lasers that operate in the required wavelength.